In recent years, there has been a large demand for a next generation optical transmission system with an enlarged transmission capacity as transmission traffic has increased.
To achieve the above-described demand, it is important to employ various modulation methods with superior Spectral Efficiency, Optical Signal Noise Ratio (OSNR) tolerance, and nonlinearity tolerance, rather than Non Return to Zero (NRZ) that has been applied in the conventional system.
Among the above-described methods, a multi-value phase modulation method such as (Differential) Quadrature Phase-Shift Keying: (D) QPSK modulation method is popular as a modulation method of the next generation optical transmission system. The multi-value phase modulation method has features such as High dispersion tolerance, High Polarization Mode Dispersion tolerance, and a narrow spectrum. As a technique for realizing further improvement of the features (OSNR tolerance and chromatic dispersion tolerance) of the multi-value phase modulation method, there is a digital coherent receiving method as a combination of coherent reception and digital signal processing.
A digital coherent receiver includes, for example, an optical front end, which has an optical hybrid and a Local Oscillator Source, and a Digital Signal Processor (DSP) performing digital signal processing.
That is, an optical signal output from the optical front end is converted into an electric signal by a photoelectric converter, is converted into a digital signal by an Analog-to-Digital Converter (ADC), and is then input to the DSP.
The optical signal output from the optical front end is output as an optical signal of each channel (the output signal is output as an I signal and a Q signal. The I signal and the Q signal of each polarization component that is mutually orthogonal is output if polarization multiplexing is applied.). As for the optical signal of each channel, the DSP may receive the signal that is converted into a digital signal.
For example, Japanese Laid-open Patent Publication No. 11-161275 discloses a technique related to the above-described technique.
In the above-described digital coherent receiver, a skew (delay time difference) may be generated between the digital signals of each channel that are input to the DSP. This skew is assumed to be caused by an electric line forming a path of each channel between the optical front end and the DSP and by individual differences of elements of an amplifier or the like. The skew may affect the quality of signals that are to be reproduced by the DSP.
An aspect of the present embodiments is to compensate a skew generated on a path of each channel between the optical front end and the processor performing processing on digital electric signals.
Another aspect of the present embodiments is to increase a margin of the skew with respect to the digital electric signal corresponding to each channel signal input to the DSP to maintain the quality of the signals to be reproduced by the processor.
The present embodiments have not only the above-described aspects but also advantages obtained by each of the configurations or the operations described in the embodiments. Another aspect of the present embodiments is to have advantages that are not obtained from the conventional technique.